test: add unit tests for extractMcpJson

Cover extracting the client id and parsed payload from a json:mcp block, and returning null when absent (app/mcp/utils.ts).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: TIANQIAN1238

test/extract-mcp-json.test.ts

@@ -0,0 +1,15 @@
+import { extractMcpJson } from "../app/mcp/utils";
+
+describe("extractMcpJson", () => {
+  test("extracts the client id and parsed mcp payload", () => {
+    const content = '```json:mcp:client-1\n{"tool":"x"}\n```';
+    expect(extractMcpJson(content)).toEqual({
+      clientId: "client-1",
+      mcp: { tool: "x" },
+    });
+  });
+
+  test("returns null when there is no mcp block", () => {
+    expect(extractMcpJson("no mcp here")).toBeNull();
+  });
+});